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|
{-|
Use this module if you like to write simple scripts with 'String'-based
errors, but you prefer to use 'EitherT' to handle errors rather than
@Control.Exception@.
> import Control.Error
>
> main = runScript $ do
> str <- scriptIO getLine
> n <- tryRead "Read failed" str
> scriptIO $ print (n + 1)
-}
module Control.Error.Script (
-- * The Script Monad
Script,
runScript,
scriptIO
) where
import Control.Exception (try, SomeException)
import Control.Monad (liftM)
import Control.Monad.IO.Class (MonadIO(liftIO))
import Control.Monad.Trans.Either (EitherT(EitherT, runEitherT))
import Control.Error.Util (errLn)
import Data.EitherR (fmapL)
import System.Exit (exitFailure)
-- Documentation
import Control.Monad.Trans.Class (lift)
import System.IO (stderr)
-- | An 'IO' action that can fail with a 'String' error message
type Script = EitherT String IO
{-| Runs the 'Script' monad
Prints the first error to 'stderr' and exits with 'exitFailure'
-}
runScript :: Script a -> IO a
runScript s = do
e <- runEitherT s
case e of
Left e -> do
errLn e
exitFailure
Right a -> return a
{-| 'scriptIO' resembles 'lift', except it catches all exceptions and converts
them to 'String's.
Note that 'scriptIO' is compatible with the 'Script' monad.
-}
scriptIO :: (MonadIO m) => IO a -> EitherT String m a
scriptIO = EitherT
. liftIO
. liftM (fmapL show)
. (try :: IO a -> IO (Either SomeException a))
|
